Enhanced sensitivity in mass spectrometry can be achieved by increased efficiency of ionization. An instrument has been modified for negative chemical ionization mass spectrometry which detects electron capturing molecules with 10-1000-fold improved sensitivity. BIBLIOGRAPHIC REFERENCES: Markey, S.P., Lewy, A.J., Zavadil, A.P., Poppiti, J.A., and Hoveling, A.P.: Quantitation of biogenic amines and metabolites using negative ion chemical ionization. Proc. 25th Ann. Conf. on Mass Spectrometry and Allied Topics, Washington, D.C., 1977. (In press.) Tate, R.L., Barden, L.K., Schultz, A.R., and Markey, S.P.: Selected ion recording minicomputer data system for magnetic deflection mass spectrometers. Proc. 25th Ann. Conf. on Mass Spectrometry and Allied Topics, Washington, D.C., 1977. (In press.)